STAT1-/- mammary tumor cell lines SSM2 and SSM3 express estrogen receptor-alpha (ERα), PR-A, and PR-B. (A) SSM2 and SSM3 expressed nuclear ERα, similar to the human ERα+ breast cancer cell line MCF7. In contrast, NMuMG and SSM1 did not exhibit nuclear ERα staining. Scale bars = 40 μm. (B) SSM2 and SSM3 are positive for ERα expression, but SSM1 expressed a very low level of ERα, by Western blot analysis. NMuMG exhibited no detectable levels of ERα. (C) SSM2 and SSM3 expressed PR-A and PR-B, which are derived from alternative PR promoters and are target genes for ERα signaling, suggesting that SSM2 and SSM3 were estrogen-responsive. In contrast, NMuMG and SSM1 did not display functional ERα signaling. DAPI, 4'-6-diamidino-2-phenylindole; SSM, spontaneous STAT1-/- mammary (epithelial tumor cell line).